There are various methods for preparing difluoro-carboxylic and perfluorocarboxylic acids.
Fluorinated derivatives of aliphatic nature, that is to say the fluorinated derivatives in which the fluorine is borne, at least partly, by an sp3 carbon, are generally obtained by an exchange of a halogen atom with a fluorine atom. This exchange generally takes place using hydrofluoric acid or else salts of hydrofluoric acid.
Another preparation process described in EP-A 1 137 615 lies in a selective hydrodehalogenation process which consists in substituting a halogen atom heavier than fluorine with a hydrogen atom.
This process is defined in that it comprises a step of hydrogenation of a substrate comprising a carbon atom of sp3 hybridization bearing an electron-attracting group and at least one fluorine atom and a halogen atom heavier then fluorine, in a basic aqueous medium and in the presence of a group VIII metal, preferably Raney nickel.
By way of examples, it is stated that sodium difluoroacetate is obtained with an excellent yield of greater than 90% by hydrogenation under pressure, from chlorodifluoroacetic acid, in sodic solution and in the presence of Raney nickel.
Thus, at the end of the reaction, an aqueous medium is obtained that comprises the sodium salt of difluoroacetic acid, sodium chloride formed during the reaction and an excess of sodium hydroxide.
The problem that is faced is that it is difficult to separate the sodium salt of the difluoroacetic acid formed during the reaction since it is a water-soluble product which is in the presence of other salts in the medium which makes the separation by filtration impossible.
Generally, the conventional technique used by a person skilled in the art is to separate said acid by an acidification followed by an extraction which involves using an organic solvent in which said acid is soluble. The difficulty is in identifying solvents that allow the solubilization and extraction of the acid from an aqueous phase.
To overcome this drawback, the present invention proposes a process that makes it possible to get round the aforementioned drawbacks.